The present invention relates to a method of producing an integral biaxially-molecularly-oriented plastics material mesh structure, of the type comprising: providing a plastics starting material having a thickness of not less than about 2 mm at its thickest point and having a pattern of holes defining strand-forming zones between respective adjacent holes, and notional junction zones between the strand-forming zones; stretching the starting material in a subsidiary direction to stretch out, thin down and orient subsidiary strand-forming zones to form subsidiary oriented strands; and stretching the material in a main direction generally at right angles to the subsidiary direction to stretch out, thin down and orient main strand-forming zones to form main oriented strands extending generally at right angles to the subsidiary strands, the stretching being continued until the thinning down extends right through the notional junction zones to aligned main strands on the other sides of the notional junction zones and extends around crotches to respective subsidiary strands so that in the crotches the orientation is in the direction running around the respective crotch, thereby forming oriented junctions with oriented crotches connecting respective main and subsidiary strands. The resultant main direction stretch is substantially greater than the resultant subsidiary direction stretch. At each junction, there is a central or thicker zone, which normally has thinned down less than the mid-points of the main strands. There are zones on either side of the central or thicker zone, which are normally thinner than the junction centre.
The present invention also relates to a biaxially-molecularly-oriented integral plastics material mesh structure of the type having a thickness of not less than about 1 mm at its thickest point, and having substantially greater strength in a main direction than in a subsidiary direction, the mesh structure comprising: main oriented strands extending in the main direction; subsidiary oriented strands extending in the subsidiary direction; and oriented junctions between respective main and subsidiary strands, respective main and subsidiary strands being interconnected by oriented crotches with the orientation in the direction running around the respective crotches, there being at each junotion a thicker zone which is substantially thicker than zones on either side thereof each on the axis of a respective subsidiary strand, and which thicker zone has a substantially greater dimension parallel to the main direction than that parallel to the subsidiary direction.
The mesh structures of the invention are for applications in which the main direction is the machine direction, and in which the main tensile force will be applied in the machine direction, and the mesh structures will have substantially greater strength in the machine direction than in the transverse direction; for instance, in a composite civil engineering structure the mesh structure can be embedded in soil and attached to a vertical wall facing, when the main tensile force will be at right angles to the facing. The main strands are those that will take the main tensile force, the transverse strands being generally at right angles to the main strands.
It is desirable to orientate as much as possible in the machine direction, but the degree of stretch that can be applied in the machine direction is limited by the risk of forming junctions which can crack or split in the machine direction on bending in the transverse direction, or of forming junctions which have marked dips in their central part. Such dips can significantly reduce the transverse direction strength of the mesh structure.
U.S. Pat. No. 4,374,798 and EP-A-0 418 104 disclose methods and mesh structures of the type referred to.